1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and particularly to a method for manufacturing a semiconductor device having a multilayer wiring structure formed by using low dielectric constant insulating materials (Low-k materials) for an interlayer insulating film provided between wirings.
2. Description of the Related Art
In recent years, as high integration in semiconductor devices progresses, miniaturization and high densification of wirings has advanced and as a result, a wiring length increases. For wiring materials, aluminum is conventionally used, however, wiring delay accompanying the miniaturization of wirings becomes a problem. Therefore, a method of using copper in place of aluminum is nowadays performed in many cases. When using copper as wiring materials, it is difficult to transfer a wiring pattern to copper itself differently from aluminum. Therefore, a damascene technique is normally used in which a predetermined opening pattern is first formed on an interlayer insulating film and copper is buried therein to form vias and copper wirings. The damascene technique includes a single damascene technique and a dual damascene technique. The single damascene technique is a technique where a copper wiring and a via for connecting between the upper and lower copper wirings are formed separately. The dual damascene technique is a technique where a copper wiring and a via are integrally formed simultaneously.
Furthermore, the following method is also performed to avoid the problem of the wiring delay accompanying the miniaturization of wirings. That is, for the sake of reduction in a capacitance between wirings, a Low-k material having a dielectric constant lower than that of a conventional oxide film is used for forming the interlayer insulating film where a via or a copper wiring is formed by the burying of copper. The Low-k materials are roughly divided into two types of an inorganic one and an organic one, and generally used properly so as to satisfy characteristics required for each device. It is known that an insulating film (a Low-k insulating film) formed by using the Low-k materials has the following characteristics. That is, the insulating film is low in the film density and is poor in the mechanical strength as compared with a conventional oxide film and therefore, when the insulating film is etched, the etched surface is readily roughened.
Many multilayer wiring structures of semiconductor devices are formed using copper for the vias or the wirings as well as using the Low-k insulating film for the interlayer insulating film, as described above. In addition, the dual damascene technique is usually used for the formation method thereof. When forming the multilayer wiring structure by the dual damascene technique using the Low-k materials, the layer structure before the formation of the opening pattern for the via or for the copper wiring is roughly divided into two types. One is a layer structure where a Middle Etch Stopper (MES) such as a SiC film is sandwiched between the Low-k insulating film layers, and the other is a layer structure where no MES is sandwiched therebetween. In these two types of layer structures, the opening pattern for the via or for the copper wiring is formed by different methods.
For example, the layer structure having the MES is formed as follows. On a lower Copper wiring, an Etch Stop Layer (ESL), a lower Low-k insulating film, an MES and an upper Low-k insulating film are laminated in this order to form the layer structure. In the layer structure of this type, the opening patterns for the via that communicates with the lower copper wiring as well as for the upper copper wiring are formed by the dual damascene technique, for example, according to the following procedures. First, a via opening that communicates with the lower copper wiring through the upper and lower Low-k insulating films, the MES and the ESL is formed by etching. Next, a wiring groove that communicates with the opening is formed by etching on the upper Low-k insulating film. Finally, the exposed MES is etched. Thus, the opening patterns for the via and for the copper wiring are formed. As described above, in the case of the layer structure having the MES, formation of the opening pattern is completed by the etching of the MES. Therefore, the exposed Low-k insulating film such as a bottom of the wiring groove is exposed to plasma during the etching for a short period of time. As a result, surface roughness hardly occurs on the film.
On the other hand, the layer structure having no MES is formed, for example, as follows. On a lower copper wiring, an ESL and a Low-k insulating film are laminated in this order to form the layer structure. In the layer structure of this type, the opening patterns for the via that communicates with the lower copper wiring as well as for the upper copper wiring are formed by the dual damascene technique, for example, according to the following procedures. First, the Low-k insulating film is etched almost to the ESL to form a via opening. Next, the Low-k insulating film is etched to a predetermined depth to form a wiring groove that communicates with the opening. Finally, the ESL is etched to expose the lower copper wiring. As described above, in the case of the layer structure having no MES, formation of the opening patterns for the via and for the copper wiring is completed by the etching of the ESL. Therefore, the exposed Low-k insulating film such as a bottom of the wiring groove is exposed to plasma during the etching of the ESL for a relatively long period of time. As a result, surface roughness readily occurs particularly on the bottom of the wiring groove.
However, from the standpoint of the device performance, the layer structure having the MES is disadvantageous in that the very presence of the MES leads to increase in the dielectric constant. Therefore, the layer structure having no MES is nowadays employed frequently.
For the sake of suppressing the above-described surface roughness of the Low-k insulating film in forming the opening pattern for one layer on the Low-k insulating film, for example, the following two experiments are heretofore performed. One experiment is that using fluorocarbon as a reactive gas, the etching of the Low-k insulating film is performed at a low speed (see, Japanese Unexamined Patent Publication No. 2004-071731). The other experiment is that using hydrofluorocarbon as a reactive gas, a reaction for forming a thin film on the surface of the Low-k insulating film and a reaction for etching the surface of the Low-k insulating film are allowed to proceed at the same time (see, Japanese Unexamined Patent Publication No. 2004-071856).
As described above, the layer structure having no MES using the Low-k insulating film has a problem that in terms of production, the surface roughness readily occurs on the bottom of the groove formed on the Low-k insulating film. Therefore, when burying copper in the finally obtained opening pattern to form the copper wiring, an uneven surface is formed between the copper wiring and the Low-k insulating film. More properly, a thin barrier metal layer having an uneven surface is formed on the Low-k insulating film prior to the burying of copper. As a result, an uneven surface is formed between the copper wiring and the Low-k insulating film.
As described above, the miniaturization of the copper wiring in the recent semiconductor devices has been advanced. Therefore, in order to bury as much copper as possible in a minute opening pattern for a via or for a copper wiring, a barrier metal film having a very thin thickness must be formed on the bottoms or side walls of the via hole and wiring groove formed on the Low-K insulating film. However, when forming a thin barrier metal film on the groove bottom having an uneven surface due to the surface roughness, also the barrier metal has an uneven surface. Furthermore, the following problems may occur depending upon the degree of unevenness. One problem is that a barrier metal film including a part having an extremely thin film thickness as compared with that in the other part may be formed on the groove bottom. The other problem is that a part having no barrier metal film may be formed on the groove bottom.
When a barrier metal film having an uneven surface is formed as described above, the following problem may occur. That is, when stress migration is caused in the copper wiring by heat stress, a large stress is applied between the barrier metal and the copper wiring. As a result, a thin part in the barrier metal is broken and copper is blown out from the broken part, whereby the copper wiring is broken.
In addition, when forming the copper wiring, seed copper is first formed on the barrier metal and then, copper plating is performed to bury copper in the opening pattern. However, when a barrier metal film having an uneven surface is formed, also the seed copper has an uneven surface and therefore, orientation in the plating copper deteriorates. As a result, the following problem may occur. That is, when a large current flows through the thus formed copper wiring, flow of the current is lowered in a part having poor orientation and excessive stress is applied thereto, whereby electromigration is caused and the copper wiring is broken.
Thus, the surface roughness of the Low-k insulating film may cause deterioration in barrier characteristics of the barrier metal or in orientation of the copper wiring. Therefore, the surface roughness may contribute to reduction in reliability of the semiconductor device.
Conventionally, some proposals are made for the purpose of suppressing the surface roughness of the Low-K insulating film. Particularly, a method of surely suppressing the surface roughness of the Low-k insulating film is required in order to realize a highly reliable semiconductor device having a multilayer wiring structure formed using the dual damascene technique.